U.S. Department of the Interior/Geological Survey Geysers Why Do Geysers Erupt? What Causes Volcanoes and by Donald E. White Hot Springs? Water from rain and snow can seep thou­ sands of feet underground. In some volcanic All rocks have small amounts of natural areas it is then heated by contact with deeply radioactivity that produce heat at very low buried hot rocks. Temperatures of this water but steady rates. Data from deep mines and can attain 400°F (204°C) or higher—much oil wells all over the world show that Earth above the temperature of boiling water at the temperatures increase with increasing depth surface. Such "superheating" is possible be­ below the surface because of this radioactive cause of the high underground water heat. Volcanoes and hot springs are also re­ pressure. When the water becomes heated, it lated to this natural heat from radioactivity. expands and rises toward the surface. Some hot systems have little if any heat other than this "normal" heat of the Near the surface where pressures become Earth. The extra heat needed for a sufficiently low, some of the water boils to system comes from (molten rock) , producing hot springs. In most hot that forms at great depths—probably at springs, the steam and the heat energy of hot depths greater than 20 miles (or about 32 water are lost by steady, quiet escape to the kilometers). In some places, magma moves surface. A few springs, however, deliver so upward to the surface to produce volcanic much energy to the surface that it cannot all eruptions; in others, however, magma stops be lost by steady escape. From time to time, short of the surface forming "hot spots" that steam bubbles become too abundant to es­ may be small or very large. Where conditions cape quietly through the water; instead the What Is a Geyser? are favorable, water that falls as rain or snow steam lifts the water, sweeping it upward and circulates underground close to these "hot A geyser is a special kind of hot spring that out of the vent. As this occurs the pressure at spots" through interconnected channelways from time to time spurts water above ground. deeper levels is lowered, boiling action in­ Figure 1.—Yellowstone and is heated sufficiently to form geysers. It differs from most hot springs in having creases, and a chain reaction is started that periodic eruptions separated by intervals leads to an eruption. These hot springs that (fig. 1). Other major geysers occur in New without flow of water. The temperature of the erupt and intermittently deliver large amounts Zealand, Chile, and the Kamchatka Peninsula erupting water is generally near the boiling of energy to the surface are called geysers. on the Pacific side of the Soviet Union. Small point for pure water (212°F or 100°C at sea geysers are also known in other countries level). Some geysers erupt less than a foot, and in several of our Western States. The and a few erupt to more than 150 feet. Some Where Do Geysers Occur? Beowawe area of north-central Nevada small geysers erupt every minute or so, but Hot springs with temperatures near boiling formerly had many small beautiful geysers, others are inactive for months or even years are rather common in many of the "hot spots" but most of these and some in , between eruptions. Contrary to popular of the world, especially in the "" of , and other countries have become opinion, most geysers are very irregular in active or recently active volcanoes around inactive in recent years as a result of ex­ their behavior, and each is different in some the Pacific Ocean. However, geysers are not ploration for (the use of respects from all others. Among the major common even in these areas, and only a few natural steam to produce electricity). geysers, only a few, such as in of the boiling springs are true geysers. The The Geysers, an important source of geo­ Yellowstone National Park, are predictable word geyser comes from the Icelandic geysir, thermal power in northern , is mis­ enough to satisfy an impatient tourist. But which means to gush or rage; Great Geysir named; the area contains no true geysers. even for Old Faithful the interval between (gayzeer) is a famous geyser in southern As in some other hot spring areas, a few eruptions varies from about 30 to 90 minutes, Iceland. A large proportion of the known shallow wells erupt intermittently, but these with an average of about 65 minutes. geysers are in Yellowstone National Park are not natural geysers. Great Geysir, the "original" geyser of Iceland

2 3 4 5 Figure 2 shows the changes in temperature that are expected with increasing depth be­ low the surface of the Earth in two kinds of areas. Curve A shows the average tempera­ ture change in the Earth with depth. Curve B represents a profile through the upper part of an area underlain at greater depth by mag­ ma, which provides the extra heat needed for a geyser system. Data from holes that have been drilled deep into hot spring areas throughout the world show that high tem­ peratures are necessary to produce geysers. The geyser systems that have been drilled al­ ways show a subsurface temperature of at least 300°F (150°C), and all that have been drilled deeply enough show temperatures of 400°F (204°C) or more. The deepest research hole drilled by the U.S. Geological Survey in Yellowstone National Park—at Norris Geyser Basin—showed 465°F (240°C) at 1,088 feet, and the temperature was still climbing. Through much of its upper and middle parts, Curve B is very close to reference Q. Curve C, which shows the boiling points of LU Figure 3.—Model of a high-temperature circulation a water with increasing depth (and pressure) system. The hot springs and geysers are fed system associated with geysers within the Earth. by water under high pressure that leaks up­ ward through cracks in this self-sealed cap. The Deep Circulation of Water Hard encrustations deposited from hot water in Geyser Systems can be seen in the geyser basins and are Figure 3 shows a model of a deep high- called sinter. temperature circulation system. The diagram Nearly all the water of geysers and hot cuts across a large area that supplies the springs originates as rain or snow that falls necessary water—perhaps 10 miles wide, 10 on the higher ground around the geyser miles long, and 3 miles deep—and includes a basins such as point A of figure 3. Most of number of geysers. (Imagine, for instance, the water flows into , but some seeps Figure 2.—Temperature variations in the Earth the diagram as applied to the entire Upper underground to great depths through inter­ Geyser Basin area of figure 1.) The geysers connected channels and open cracks in the Curve A—Normal increase in temperature with and hot springs are in the visible "skin" but rocks. Some hot spring systems have been depth are only a small part of the total system. drilled as deep as 8,000 feet. Even at these Curve B—Temperatures of upflowing water in a great depths, the water is mostly of surface large geyser system Near the area where the upwelling water reaches the surface, the hot water transports origin. Thus, we infer that the water of the Curve C—Reference Curve; temperatures that geyser-bearing systems circulates to depths water must attain to boil at various depths and deposits silica and other which in Upper Geyser Basin, Yellowstone tend to form a self-sealing cap or "lid" on the of at least 5,000 feet and perhaps more than National Park, erupts about 30° from vertical. This 10,000 feet. unique geyser is now dormant.

6 7 8 9 indicate that this tremendous flow of extra heat has continued for at least 40,000 years. This large outflow of heat, over many years, is the major reason for believing that a large magma chamber exists below the circulating water system. Water circulates in these systems because of differences in temperature. Hot water of a given volume weighs less than cold water; at 212°F (100°C), it is about 4 percent lighter in weight than at freezing point (32°F or 0°C). Water at 400°F (204°C) is 14 percent lighter, and at 600° F (about 315°C) it is 28 percent lighter than at 32°F. The cold water, because of its extra weight, pushes the hot water on­ ward and eventually upward and out of the system. Differences in altitude between points A and E of figure 3 also give a gravi­ tational advantage that provides some extra driving force. The circulating water has great differences in temperature. It is cool when it seeps un­ Steam clouds rise over Midway Geyser Basin, derground from rain or melted snow at point increases markedly as it flows from B to C; Yellowstone National Park. A. It then percolates downward many thou­ the actual increase depends on how fast it is sands of feet and is heated only slightly by the flowing and how much heat is rising upward from D to E, temperatures decrease con­ time it reaches point B. Its temperature then from the magma chamber. If the water is flow­ tinuously by boiling as the extra heat in the ing rapidly or if the supply of heat is too hot water converts some water to steam (see small, the temperature at point C is less than Curves B and C, figure 2). At the surface of the in Upper Geyser Basin, Yellowstone 300°F (150°C) and the system cannot sustain Earth at sea level, the temperature must be National Park, erupts through a vent in its sinter cone. natural geysers at the surface. 212°F (100°C) for water to boil. At the alti­ The vast differences in the temperature of tude of Yellowstone National Park (because A magma chamber is shown at the bottom the circulating water are possible because of lower air pressure at about 7,000 feet of figure 3; the heat from this magma drives pressure increases with depth. Although the altitude), water boils at only 199°F (93°C). the system. A small amount of volcanic water water at point C may be heated as high as Geysers exist because water can be heated or steam (small arrows) is shown rising from 500°F, it will not boil because of the tremen­ to higher temperatures where pressure is this magma, but research has shown that dous depth and water pressure (see high (as in a pressure cooker), but this very volcanic water forms less than 5 percent of reference Curve C of figure 2). The hot water hot water must give off its extra heat as steam the total water involved in geyser action. rises upward from point C (fig. 3) because when the water rises and pressures de­ A tremendous supply of heat is essential. the heat has made it lighter and the cold heav­ crease. Thus, this rising water carries its own Recent measurements by the U.S. Geological ier water moving in from the side is driving it extra energy required for geyser eruptions. Survey show that the total heat flowing from onward. As it rises, the pressure decreases. Having followed the water as it circulates Upper Geyser Basin is at least 800 times The water eventually rises to point D, through a large hot spring system and rises more than the heat flowing from a "normal" The "fountainlike" Little Whirligig Geyser in Norris where pressures have dropped sufficiently toward the surface, let us focus on a single area of the same size! Geologic studies also Geyser Basin, Yellowstone National Park for steam bubbles to form. With further rise geyser and its mechanics of eruptions.

10 11 12 13 steam bubbles is heated as the steam con­ The "Plumbing System" of reservoir that can ordinarily be measured denses; finally, the boiling temperature for with scientific instruments is directly below Individual Geysers the existing pressure is reached. New steam the surface vent. that enters the tubes can no longer condense Figure 3 shows a large system that sup­ Temperatures deep in the main tubes of in cooler water; these steam bubbles start to plies hot water to many hot springs and geysers are generally too low to trigger rise, and they expand continuously upward geysers. The local reservoirs or "plumbing eruptions. The triggering mechanism thus as pressures decrease. systems" of individual geysers are in the occurs either near the tops of the geyser uppermost shallow parts of the large system, tubes, where the rapidly upflowing water is At some critical time and place that differ within a few feet to perhaps several hundred hot enough to boil, or in the deep inacces­ for each geyser, the steam bubbles form and feet below the surface. Figure 4 is an enlarge­ sible parts of the "plumbing" system where expand so fast that they can no longer flow ment of only a small part of figure 3 and much hotter water is flowing into the geyser upward individually through the liquid water; shows a hot spring that does not have the reservoir, as at points A or B of figure 4. instead, the water is swept upward along with conditions necessary for eruption and a Because this inflowing water may have a the bubbles. The effect of the many expand­ single geyser. temperature at or even slightly above the ing bubbles is now like a moving plunger, A geyser reservoir commonly consists of a boiling point (Curve C of figure2) for its forcing the water upward through the geyser rather large, nearly vertical tube that is open depth (because of extra pressure), some of the tube. At first the frothing water overflows to the air, but that leads down through nar­ inflowing water may flash into steam as the quietly or in individual spurts, and then, like rower and narrower "roots" that are the feed­ pressure abruptly drops to that of water in a giant water gun, it may erupt explosively. ing channels. The only part of a geyser's the geyser tube. The liquid water near these The reason for this increased rate of dis­ charge is that, as water flows out from the top of the geyser tube, the weight of steam that is forming is much less than the weight of the displaced water. The pressure on the water in Upper Geyser Basin, Yellowstone at deeper levels therefore decreases. Much National Park, is one of the few geysers whose erup­ tions are somewhat predictable. of this deep water was already near the boil­ ing temperature; with lowering of pressure it the supply of water lost in its previous erup­ starts to boil, more water flashes into steam tion, but can erupt only intermittently. During at progressively deeper and deeper levels, an eruption, the local reservoir is partly and a chain reaction is started. emptied or "overdrawn," just like a In most geysers, the climax of eruption account when withdrawals exceed deposits. occurs very soon after the eruption starts, when the supply of water is greatest and tem­ The Geyser Cycle peratures are highest. Especially during this Now we can understand the parts of a early stage, water erupts from the open tube complete geyser cycle. The eruption is, of much faster than new hot water can flow into course, the spectacular part. As the main the geyser through its narrow feeding cracks eruption ends, some geysers first "run out of and channels. The tubes are large and open water" and then have a steam phase. Other only near the surface, as in figure 4. At geysers first run out of extra heat (energy) deeper levels, the cracks and channels are while water is still abundant and stands high partly filled with minerals deposited from the in the geyser tubes. In the geysers that first hot water. The flow of water into each local run out of extra heat, steam bubbles form geyser reservoir is slow, relatively continu­ more and more slowly as the eruption de­ ous, and under high pressure. Through this creases in vigor. Finally, a moment is slow, continuous feed, a geyser gains back reached when the few steam bubbles again Figure 4.—Model of geyser tube showing reservoir of pore spaces and narrow feeding channels

14 15 16 17 rise slowly through the water without sweep­ surges. During the steam phase of a geyser outside the cone or pool, and how much flows eruptions but not in others. For this reason, ing the water up to the ground surface, and eruption, pressure of water draining in from back into the geyser tube, thus conserving the many geysers have two or more different the eruption ends. A recovery period is then the reservoir margins may nearly balance the water supply but requiring more heat for the intensities of eruption, as Steamboat Geyser necessary—generally much longer in time pressure of steam and other gases. The pres­ water to erupt again. Connections to other in Norris Geyser Basin has had since 1961. than the eruption. The recovery consists of sure of the collecting water generally be­ nearby springs and geysers may also drasti­ two aspects: recovery of water and, in part comes dominant after a single eruption, but cally affect the activity and predictability of a How Geysers Are Born, contemporaneously, recovery of heat (or the balance may be close enough for many geyser. and Why They Die energy). The recovery of water generally geysers to erupt again in two or more closely An eruption can be confined essentially to occurs first. This fact is most easily seen in spaced stages. a single central tube with narrow feeding Most geysers probably start when new geysers like Riverside in Upper Geyser Basin Other differences between geysers depend channels, as in Vixen Geyser of Norris Gey­ cracks or channels are formed by earth­ that, after eruption and quiescence, have a on how much of the erupting water falls ser Basin, or the reservoir can extend down­ quakes. The 1959 Hebgen Lake period of preliminary overflow. Water starts ward or outward to include other almost just west of Yellowstone National Park in to from the vent minutes or even separate parts. These deeper parts may re­ southern Montana created at least one com­ hours before the supply of heat has become gain enough energy to take part in some pletely new geyser, Seismic, but spectacular great enough to trigger an eruption. In such geysers, the subsurface channels and open spaces of the reservoir fill with water before any appears at the surface. Temperature measurements show that the first water to overflow is generally the coolest. As flow con­ tinues, the temperature also increases. Water at low temperature is being flushed out of the reservoir while much hotter water is flow­ ing in. With this loss of relatively cool water and gain of hotter water, the geyser is "reloading," or recovering the energy it needs to erupt again.

Explanation for Differences Among Geysers, and Complex Eruptions

The general explanation of a geyser cycle given above may clarify why geysers erupt and why they stop, but it does not help in understanding why each geyser differs from all others. The eruptions of some geysers are very complex and difficult to understand. For example, Grand Geyser of Upper Geyser Basin in Yellowstone Park has spectacular separate surges. Each new surge starts sud­ denly about 2 minutes after the previous Steamboat Geyser in Norris Geyser Basin, Yellowstone surge has ended. Each complete eruption National Park, is considered to be the largest and most powerful geyser in the world. The height of this Seismic Geyser in Upper Geyser Basin, Yellowstone National Park, erupts through a narrow crack that developed during Montana's consists of 2 to more than 30 separate eruption was estimated to be about 300 feet. Hebgen Lake earthquake of 1959.

18 19 20 21 changes also occurred in many other pre­ thus effects of geyser action, and are not the existing ones. Numerous springs not known original causes! Some geysers become so to have erupted previously became active, vigorous that they "commit suicide" by form­ and some small geysers became major ones, ing very large tubes and pools; the extra such as Sapphire Geyser in Upper Geyser energy can then be lost by the steady Basin. Most changes were almost immediate processes displayed by ordinary hot springs. but others developed over several years. Most of the large hot pools in Yellowstone In figure 4 the geyser tube and other large were at one time geysers. Some of these nearby open spaces (such as near point A) probably "committed suicide" by vigorous probably formed by enlargement of a narrow enlargement but others may have become in­ crack, just as Seismic Geyser developed as active by of minerals in their feed­ it increased in vigor, ripping off and ejecting ing channels (hot spring of figure 4). With rock fragments that may be seen in the slower "feed," the smaller extra heat can photograph, along with trees killed by the then be lost by circulation, evaporation, and increased activity. The large open spaces are the quiet rise of steam bubbles through the water. Probably in time, all geysers change their behavior and eventually become inactive. Very small changes in the channels and pat­ terns of flow can result in major changes in behavior. The western major vent of Minute Geyser in Norris Geyser Basin formerly erupted to heights of nearly 100 feet, but the tube was filled with rocks by careless or curious tourists, and the geyser no longer erupts. Geysers are rare and beautiful. Let us treasure and preserve these wonderful dem­ onstrations of nature's energy!

This publication is one of a series of general interest publications prepared by the US Geological Survey to provide information about the earth sciences, natural resources, and the environment To obtain a catalog of additional titles in the series "Popular Publications of the US Geological Survey," write: Eastern Distribution Branch, or Western Distribution Branch US Geological Survey US Geological Survey 604 South Pickett Street Box 25286, Federal Center Alexandria, VA 22304 Denver, CO 80225

Minute Geyser in Norris Geyser Basin, Yellowstone National Park, is no longer active because of debris that accumulated in its main vent.

22 23 OU.S. GOVERNMENT PRINTING OFFICE: 1984-421-618/10002